Significant advances have been made involving two groups of retroviruses that have been implicated in the etiology of mammary neoplasia: the type-B murine mammary tumor viruses (MMTVs) and the type-D retroviruses of primates. Recent findings include: (a) The isolation and characterization of MMTV host range variants that efficiently infect murine and heterologous cells in vitro; (b) the development of the first interspecies radioimmunoassays for several of the major structural proteins of type-B and type-D retroviruses; (c) the identification and characterization of new viruses that are immunologically related to MMTVs; (d) a demonstration of the diversity of mammary tumor viral genes within the genus Mus, the species Mus musculus, and the strain C3H; (e) the demonstration that the primate placenta is a good source for the detection of type-D retroviral RNA and DNA; (f) the use of relaxed hybridization conditions to detect two distinct classes of proviral sequences, related to MMTVs, in the DNAs of rats; (g) the demonstration that certain subsets of MMTV genes segregate as a single Mendelian entity; and (h) that molecular hybridization can be used as a sensitive and specific assay to detect lung micro-metastases in mice bearing spontaneous mammary tumors.